WebSphere Application Server V7.0: Concepts ... - IBM Redbooks

garbage collection. The garbage collection for a JVM heap requires all pages of
the heap to be in exclusive access and in real memory. If any pages are not in the
real storage they first need to be paged in.
As an example look at the environment from Figure 14-3 on page 426. This
includes a deployment manager and one application server (with three servants).
Assume each control region heap has a maximum of 256 MB and each servant
region a maximum of 512 MB defined. Then the real storage that the WebSphere
Application Server for z/OS needs is 2.5 GB. This does not consider that there
might be other middleware installed in the z/OS image.
Make sure that the LPAR that is used for the installation has enough real storage
defined. Also keep in mind to add storage for the operating system and other
applications running in this LPAR, like DB2, CICS, and so on.
Health check: We suggest to monitor your system and check for swapping. If
you experience swapping, this will have a performance impact.
Heap sizes (min/max) defined
Usually the z/OS version will need smaller maximum heap sizes than the
distributed version, because it has specialized heaps in its structure, as
described in 14.1.4, “Structure of an application server” on page 422.
This is of interest when migrating an application from another platform to
WebSphere Application Server for z/OS V7.0. Often, the memory size from the
distributed environment is carried on from the distributed environment and
reused for the controller and servant regions settings. While this may be a waste
of memory resources, it can affect the performance as well. If the heap is sized
too large, then the Garbage Collection will run less often, but if it runs, it takes up
more time. This might reduce the general throughput.
Important: If an application is migrated to WebSphere Application Server for
z/OS V7.0 from another operating system family, perform a verbose Garbage
Collection analysis. This will allow you to size the heap to a good minimum
and maximum value, so that the performance is good and no resources are
wasted.
458 WebSphere Application Server V7.0: Concepts, Planning, and Design